As high-bandwidth fiber-optical communications technologies become more and more mature, and the application cost declines year by year, fiber access networks gradually become more competitive for next-generation broadband access networks. Among fiber access networks, passive optical networks (PONs) are especially competitive. Generally, a specific structure of a passive optical network system may be referred to FIG. 1, the passive optical network system includes an OLT (Optical Line Terminal, optical line terminal) located at a central office, an ODN (Optical Distribution Network) for branching/coupling or multiplexing/demultiplexing, and multiple ONUs (Optical Network Units) located at user ends. The PONs may be categorized into different types according to different implementations, where WDM-PON systems using the WDM technology gain much attention owing to advantages such as large bandwidth capacity, and information security of quasi-point-to-point communication. However, the WDM-PON has high cost compared with fiber access networks which use TDM (Time Division Multiplexing) technologies such as an EPON and a GPON, where the excessive cost of a light source is an important factor causing the excessive cost of the entire WDM-PON system.
The WDM-PON uses an AWG (Arrayed Waveguide Grating) or a WGR (Waveguide Grating Router) at the user end, wavelengths on AWG ports or WGR ports connected to user end ONUs are different, and therefore, different ONUs need to use optical transceiver module with different wavelengths, which are named as colored optical modules in the field of optical communications. The use of the colored optical modules in the ONUs may render the ONUs failed to be commonly used; and at the same time, bring difficulties to service distributions of an operator as well as a storage problems. In order to solve the problem of the colored ONUs, a proposal of WDM-PON colorless light source is raised in the industry, that is, an ONU transceiver module is independent of a wavelength, an emission wavelength may be automatically adapted to the wavelength of a connected AWG or WGR port, so that the ONU transceiver module can achieve plug-and-play on any AWG or WGR port.
In order to achieve the colorless ONU transceiver module of the WDM-PON, multiple solutions are proposed in the industry, including a self-seeding fiber laser. Referring to FIG. 2, a schematic diagram of a WDM-PON system using a self-seeding laser is illustrated. In the WDM-PON system, after a multi-longitudinal-mode optical signal emitted by a self-injection locking laser of a user end ONU is filtered by an AWG at a remote node (RN), only an optical signal of a corresponding wavelength may permeate the RN-AWG and enter a partial reflection mirror (PRM) disposed at a trunk fiber. Due to the partial reflection mirror, a part of light is reflected back and re-injected into the self-injection locking laser. A gain cavity of the self-injection locking laser amplifies the light reflected back again and then transmits the amplified light, and such round-trip oscillation is performed for multiple times. Accordingly, the self-injection locking laser and the partial reflection mirror cooperatively form an external cavity self-seeding laser, and a laser resonance oscillation cavity is formed in between and outputs a stable optical signal. After uplink data of the ONT being modulated to the optical signal, the uplink data of the ONU may further pass through the trunk fiber, and be demultiplexed by an AWG at a central office (CO) and then output to a corresponding receiver (Rx) of an OLT. Similarly, a downlink optical signal emitted by the OLT is demultiplexed by the RN-AWG, and then output to a receiver of a corresponding ONU.
Although the foregoing solution may achieve the colorless optical transceiver, the AWG needs to undertake functions of intra-cavity filtering for a transmitting end and demultiplexing for a receiving end at the same time. As for the transmitting end, each channel of the AWG is used as an intra-cavity filter of the self-seeding laser, which requires that a filtering curve of the AWG channel has narrow bandwidth and has a maximum transmissivity at a central wavelength of the channel. As for the receiving end, the AWG functions as demultiplexing, which requires that the AWG channel has a wide bandwidth and a transmission curve is flat in the channel. Due to the two contradictive requirements, the optical transceiver and the WDM-PON system that are based on the self-seeding laser have limited performances, and fail to meet the demand in practical application.